Electrically insulated element



June 6, 1944. P. ROBINSON ELEGTRICALLY INSULATED ELEMENT Filed Dec. 3,1941 PRESTON ROBINSON INVENTOR.

1 1 A a A a a ATTORNEYS Patented June 6, 1944 UNITED STATES PATENTOFFICE 2,350,822 ELECTRICALLY INSULATED ELEMENT Preston Robinson, Wiiliamstown, Mass assignor to Sprague Specialties 00., North Adams, Mass,a corporation of Massachusetts Application December 3,1941, Serial No.421,524

Claims.

The present invention relates to electrically insulated elements, andmore particularly to new and improved type of coils and to the method ofmanufacturing same. 7

My invention will be described in connection with coils as used forelectromagnets, magnet poles of rotary electrical machinery and thelike, the coiled wire of which is a conductor coated with athermoplastic vitreous insulatin medium and the adjacent turns andsuperimposed layers of the coil are bonded together by fusion of saidinsulating medium. It should, however, be understood that my inventionis in no way limited to such use.

Coils in which the insulation of the conductor consists of a fusedvitreous material have already been proposed. Such coils, however,proved unsatisfactory because the differential thermal expansion ortheir component parts caused high internal stresses in operation, andthe vitreous materials used lacked flexibility to withstand suchstresses without chipping and flaking. As a result, such coils rapidldeteriorated in operation, particularly if the coil was embedded in asolid matrix of the insulating medium.

The object of the present invention is to provide a new and improvedcoilin which the adjacent turns and superimposed layers of the coil arebonded together with an insulating medium.

Another object of the invention is to provide a coil in which theinsulating coating of the conductor forms an elastic bond betweenadjacent turns and layers of the coil, and in which internal stressesare substantially or altogether eliminated.

These and other objects of the invention will appear as thespecification progresses.

According to my invention, a coil consists of a wound conductor providedwith. an integral oating, and adjacent turns and superimposed layers ofthe coil are by interfusion intimately joined to each other by a bondformed of the coating, which bond is restricted to portions of theadjoining surfaces of the turns, whereby cavities or interstices areformedbetween adjacent turns and the layers, which extend throughout theentire coil.

By restricting, in accordance with the invention, the bond only toportions of adjoining surfaces of adjacent turns-and superimposed layersof the coil, a compact and rigid and at the same time highly flexiblestructure is obtained, which precludes undue internal strains in the.coil.

I preferably use for this purpose a coating conglass frit, and asuitable binder.

To obtain the best results, the vitreous mate I rial should constitute amajor proportion (80% sisting of a vitreous material or enamel in the ormore, by weight) of the coating. A coating of this type and the methodof applying same to a conductor, is described in my copendingapplication Ser. No. 325,043, filed March 20, 1940.

As described in said application, coatings showing high flexibility areobtained by the use as a binder of a compound selected from the group01' organic materials known as elastomers." Such binder materials ifdispersed in a suitable suspension medium-to which the frit particlesare addedare conditioned to exhibit high adhesive forces, and which tofully realize, the ground frit and the elastomer are depositedsimultaneously by cataphoresis on the conductor.

ample, the acrylic ester resins, such as those known under the tradenames of Appretan A" and Corial Bottom.

The coated conductor is then wound into a coil the coil, which bringinto contact coatings of adjacent turns and cause an interfusion andflexible bonding of only portions of such adjacent coatings, thusproviding for interstices between turns and layers.

The degree of compression to be applied depends on a number of factors,such as the resiliency of the conductor, the plasticity of the coat-'ing, the degree and duration of the heat treatment, etc. To obtain theobject of the invention, the compression during the ,heat treatmentshould be sufficiently high to cause adjoining coatings to intermingleand coalesce but not so high as'to cause the coatings to flow into eachother to the extent as to prevent formation of the desired interstices;the formation of such inance with another terstices or voids being anessential part of my invention.

The proper compression, which depends on the above enumerated variousfactors, may require a somewhat critical adiustment of the value or thecompressive force. To avoid this, and in accordfeature of the invention,I embody within the coating particles of a nonthermoplastic insulatingmaterial, which do not melt or fuseor lose their identity during theheat treatment interfusing the adiacentportions orthe coating of thecoil. Particularly suitable for this purpose are particles of refractoryinsulating materials, such as silica, mica, aluminum oxide, magnesiumsilicate and the like.

In practice the above refractory materials prove to be somewhat solublein the coating when it liquefles, the degree of solubility depending onthe concentration and size of the particles and the temperature andduration of the heat treatment. To insure that a sufficient amount ofdiscrete particles remain in the coating after the heat treatment, thecoating should contain a large portion of refractory particles, forexample to 30% or more by volume, and such particles should be of theorder of 200 mesh in size.

I have found that the desired bonding between the surface of adjacentcoatings may be obtained by applying compressive forces along only oneaxis of the coil, for example, axially, (along the longitudinal axis ofthe coil), which compression may be effected by applying compressiveforces at the two ends of the coil.

It should be noted that in case of axial compression the bonding isprimarily between the coating of adjacent turns; however, all of thelayers in turn are biased against each other by the inherent flexibilityand resiliency of the structure, and a bonding along the complementaryaxis is obtained.

In certain instances it may be desirable to apply both axial and radialcompression to the coil, in which case, however, the degree ofcompression along either axis is usually less than if axial compressionalone is used. In general, compression along both axes, I have found,brings about a greater degree of uniformity oi! bonding in the coilstructure.

Fusion of the coil under proper compression brings about a compact,coherent and stress-.

resistant structure in which adjacent portions of the wire coating areintimately bonded together. At the same time the bond is restricted toonly small portions of the adjacently disposed coatings, and a coilstructure comprising a mult plicity of voids or interstices betweenturns is obtained.

Furthermore, in view of the small area of bonding and the consequentlarge percentage of voids, a coil with a low distributed electricalcapacity is obtained, which makes the coil particularly suitable forspecial applications, for example, for use at high radio frequencies. 7

In the drawing forming part of the specification,

Figure 1 illustrates a jig or fixture for compressing a coil during theheat treatment thereof, in

accordance with one embodiment of the inven-- tion,

Fig. 2 is an enlarged view in cross section of a portion of a coil madein accordance with the invention, showing the bond and the intersticesprovided between turns and layers of the coil.

Referring to Figure l, the fixture there shown, which is adapted for theaxial compression of a coil during its heat treatment to interfuse theadjacent portions of the coating, comprises a hollow core l3 consistingpreferably of a refractory ceramic material, such as SteatiteJIsolantite, porcelain or the like. At one end of the core there isprovided a flange ll preferably of ceramic material and provided with acountersunk bore 2|, which flange I4 is positioned on the core by a boltll passing through the core l3, and having a head piece 20 positionedwithin the bore 2| and a nut IS with an interposed washer i6 pressingagainst the end of the core.

Disposed over the core l3 and slidingly engaging the same is collar 15,preferably of ceramic material. The coil to be subjected to axialcompression, shown in the drawing as I0, is disposed within a spacebounded by the flange l4 and the collar l5. For taking out the beginningof-the coil, the flange I4 is provided with a radial recess shown at 12.

.Positioned on the collar I5 is an annular metal body is serving as aweight for compressing the coil ill in accordance with the invention.

In making a coil in accordance with the invention the followingprocedure may be used: A wire conductor is provided with an integralcoating, preferably of the type described in my copending applicationSer. No. 325,043, referred to above. Such a coating may consist of amixture of vitreous and refractory particles and an elastomer binder,for example 68% of porcelain enamel particles, 17% silica particleshaving a size of the order of 200 mesh, and 15% by weight of theelastomer Corial Bottom.

The coated wire is then wound in conventional manner into a coil of thedesired form. This may take place on a suitable mandrel of a windingmachine, from which it is then transferred to the core l3, or core l3may itself be used as the mandrel. In either case, as appears from thedrawing, the coil occupies only a portion of the core l3.

The coil i0 having been thus provided on the core, the remainingcomponents, i. e. flange 14, collar l5, weight 19 and bolt l1, are addedto assemble the fixture into the structure above described.

The compression of the coil is efiected by the annular member 19 and thedegree of compression may be adjusted by correspondingly varying theweight of this member.

It should be well understood that the core I3 instead of beingcylindrical in shape may have other geometrical cross sections dependingon the desired shape of the coil.

The so assembled fixture is then placed in a suitable oven for heattreatment to interfuse limited portions of the coating; said heattreatment in its preliminary stages may also serve to bake the coatingand to drive off substantially all of the binder in the coating. Thetemperature used and duration of the heat treatment depends ,on thematerial of the coating, the thickness of the coating, and thedimensions of theconductor and the coil. For example, in the case of acoil wound of a copper wire .016" in diameter having a coatingapproximately .0025 thick, which coating consists of the aboveillustrated constituents, using a porcelain core approximately .3125 indiameter and which coil consists of six (6) superimposed layers oftwenty-six (26) turns each; a heat treatment at approximately 700 C. forapproximately two (2) minutes has been found to be suitable.

It will be noted that the interfused bond between the coatings ofadjacent turns, whether these turns are those of the same layer or ofsuccessive layers, extends over only a portion of the surface of thecoatings whereby a series of interstices 32 are created which formsubstantially helical continuous ducts between the layers and turnsextending throughout the length of the coil.

It should be understood that instead of the fixture shown in Fig. 1,equivalent structures may a procedure has also the advantage ofproviding an intimate bonding of the coil to the metal carrier.

While I have described my invention by means scope of the invention.

What I claim is:

1. An electrical coil comprising a conductor wound into adjacent plasticmaterial uniformly distributed throughout the coating, bonds formedbetween the adjacent turns and superimposed layers, said bondsconsisting of interfused portions of the thermoplastic material andextending over only part of the adjacently positioned portions of saidcoating.

2. An electrical coil comprising a conductor wound into adjacent turnsand layer an integral coating of a thermoplastic insulating medium onthe conductor, discrete refractory particles uniformly distributedthroughout the coating, and bonds formed between the adjacent turns andlayers of the coil and consisting of interfused portions of the coating,said bonds extending coating, and substantially tubular and continuousinterstices extending throughout the coil.

4. In the manufacture of electrical coils the process comprising thesteps, providing on a wire 5. The method of manufacturing an electricalcoil comprising the steps, providing on a wire conductor an integralflexible insulating coating consisting of particles of a vitreousmaterial and of a refractory material and a volatilizable binder, thevitreous material constituting a major proportion of said coating andthe refractory material being present in a substantially high proportionof the vitreous material,

PRESTON ROBINSON.

